1. Field of the Invention
The present invention relates to an optical pick-up apparatus capable of reading data irrespective of a disc type, and in particular to an improved optical pick-up apparatus capable of reading data irrespective of a disc type capable of reading a certain disc among disc having different thicknesses and writing densities using only one pick-up apparatus.
2. Description of the Conventional Art
Generally, a digital video disk (DVD) is provided with a red semiconductor laser and an objective lens having a larger numeric aperture (NA). The above-mentioned DVD has six or eight times the writing data capacity of a compact disc (CD), in which compressed video and audio data can be stored therein. That is, the data corresponding to that of a movie can be stored in a disc of 120 mm.
The DVD uses a red semiconductor laser having a wave length of 635 mm or 650 nm.
Generally, if the wave length of light source becomes short, the diameter of a laser spot decreases in proportion thereto, so that both a track pitch and a minimum writing mark length can be reduced. That is, since the surface of the writing mark is in duplicate proportion to the wave length of a writing mark, overall writing area can be decreased.
Meanwhile, the spot diameter of the laser is proportional to the wave length of the light source. Therefore, if the numeric aperture is increased, without varying the wave length, it is possible to increase the writing intensity. Therefore, the numeric aperture in the optical system for a CD is about 0.45, but it is about 0.6 in the DVD.
There are three methods below of reading the disc data in the conventional DVD as follows.
The first is to simply increase the numeric aperture by a small amount. The second is to adopt a tilt angle compensation device, called a tilt servo, in the optical pick-up system, instead of increasing the numeric aperture to more than 0.52. The third is to increase the numeric aperture up to 0.6 and to reduce the laser transmitting distance through a disc plate.
The construction of a conventional optical pick-up apparatus will now be explained.
Referring to FIG. 1, the conventional optical pick-up apparatus includes a diffraction grating 2 for dividing a beam from a light source 1 into a main beam and two sub-beams for a tracking servo. In addition, the beams from the diffraction grating 2 is transmitted to an objective lens 5, provided for condensing the light on a spot of an optical disc 6, through a collimator lens 4 for outputting a parallel light through a beam splitter 3. In addition, the optical detector 8 detects a beam data signal transmitted from a sensor lens 7 for condensing the beam.
The operation of the conventional optical pick-up apparatus will now be explained with reference to the accompanying drawings.
The beam from the light source 1 is converted into a parallel light by the collimator lens through the beam splitter 3. The beam is focused by the objective lens 5 and reflected or diffracted by information writing surface on an information media. The thusly reflected beam returns on the same path and is converted into an electric signal by the photo-detector 8. That is, the beam is transmitted to the photo-detector 8 through another path formed by the beam splitter 3 through the sensor lens 7.
Meanwhile, the diffraction grating 2 and the sensor lens 7 are widely used with respect to tracking using a three beam method and to astigmatism method a focusing servo.
A high density optical disc has four times the capacity of a conventional compact disc, and the data stored therein can be reproduced using a lens having a numerical aperture of about 0.6. In this case, the aberration due to the disc inclination increases as the thickness of a disc increases. In order to resolve the above problem, the standard of a digital video disc requires the thickness of a disc to be 0.6 mm.
At this time, the optical system, as shown in FIG. 1, includes a high density optical disc having a thickness of 0.6 mm and a compact disc having a thickness of 1.2 mm. However, the above-mentioned optical system has the following problems.
For example, the beam intensity distribution at a focal point on a disc surface using and a disc having a thickness of 0.6 mm and an objective lens having a numerical aperture of 0.6 is shown in FIG. 2 as a solid line. When a beam is focused at a disc having a thickness of 1.2 mm through the objective lens 5, the beam intensity distribution is shown as a dashed line in FIG. 2 due to the spherical aberration of the lens.
That is, the beam intensity ratio of the main lobe significantly decreases, and the beam intensity of the side lobe increases, so that crosstalk of the signal written on its neighboring track of the disc increases.
In addition, the optical pick-up apparatus cannot reproduce the data stored in a disc of 1.2 mm because the sensitivity with respect to a disc tilt level is too high as shown in FIG. 1 in case the data is read using an objective lens having a numerical aperture of 0.6 mm.